1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for purifying hydrocarbon, carbon monoxide or nitrogen oxides (NO.sub.x) contained in exhaust gases exhausted from an internal combustion engines of automobiles and the like.
2. Description of the Related Art
Conventionally, inorganic porous body such as alumina or zeolite is mainly used as a support of an exhaust gas purifying catalyst for automobiles. Japanese Laid-Open Patent Publication No. 201648/1987 discloses various kinds of supports such as alumina, silica, titania and alumina-silica. Japanese Laid-Open Patent Publication No. 147153/1980 discloses A-type zeolite, X-type zeolite and mordenite-system zeolite. Japanese Laid-Open Patent Publication No. 4220/1989 discloses crystalline zeolite. Japanese Laid-Open Patent Publication No. 63552/1990 discloses composite oxides comprising titanium, nitrogen and ziroconia. Japanese Laid-Open Patent Publication No. 245850/1989 discloses an aggregate comprising ceramic fibers.
The above inorganic porous bodies have wide specific surface, acidity and heat resistance at high temperature, so they are suitable for catalytic supports. These supports are used as conventional exhaust gas purifying catalysts in such a manner that a noble metal such as platinum or rhodium is loaded on these supports.
So-called cold HC is the problem, for it is included in the exhaust gas at low temperature immediately after the engine is ignited and it is hard to be purified by catalysts. To solve this problem, there is proposed an exhaust gas purifying apparatus such as cold HC purifying system and the like which use absorbent such as zeolite for temporarily trapping cold HC (Japanese Patent Laid-Open No. 2-75327/1990 and Japanese Patent Application No. 3-49658/1991).
However, the above-described conventional exhaust gas purifying catalysts have poor catalytic activity. So, they are required to improve catalytic activity in order to purify exhaust gases in diesel engines.
In the case of the above-mentioned exhaust gas purifying apparatus, hydrocarbon components, which are trapped by zeolite as the absorbent, are limited to have relatively small molecules. In this apparatus, it is difficult to efficiently trap bulky aromatic HC such as trimethylbenzene which causes troubles in some countries as the material having the high capacity of forming ozone. This may be related to the small pore diameters for trapping in zeolite.
Zeolite has the low heat resistance, so it is hard to directly be provided in the exhaust gas system of an internal combustion engines. Therefore, zeolite should be provided in the system with a bypass or the like.
As mentioned above, in the conventional evaporated fuel absorbing apparatus and the hydrocarbon trapping apparatus of the exhaust gas purifying apparatus, there are various problems caused by the insufficient specific surfaces of absorbents for trapping hydrocarbon components, small pore diameters for trapping and the low heat resistance.